This invention relates to electrostatic discharge devices adapted for use in applying a uniform electrostatic charge to a charge retentive surface and in particular to a device which uses pin or needle electrodes for negatively charging the surface.
The most common discharge devices used for uniformly charging a charge retentive surface either negatively or positively are of two types. One type utilizes one or more bare wires positioned parallel to the surface to be charged and the other type utilizes an array of pin or needle electrodes which are supported such that the axes thereof are substantially perpendicular to the surface. The bare wire type of device is more commonly employed than pin or needle electrodes. However, a bare wire is usually less suitable for negative charging because of charging non-uniformities and it generates larger amounts of ozone.
The provision of multiple wires minimizes the non-uniformity problem but creates space problems and increases the cost of the device. The non-uniform charging with bare wire devices has been minimized by continuously applying higher voltages to a single wire to produce higher current densities flowing from the wire but this creates even greater amounts of ozone because of the higher currents. Significant amounts of ozone are objectionable for various reasons. One of the more important objections is its adverse effects on photoconductive surfaces used in xerography. Since the charging devices are very close to the surface it is difficult if not impossible to keep ozone away from the surface.
Certain features of the pin device are self-evident: it is rigid, nonbreakable, and not subject to vibrations. This leads to advantages in field maintenance. For example, the potential for photoreceptor damage due to broken corotron wires is eliminated and the potential for electrical accidents from a dangling high voltage wire does not exist. Moreover, from the standpoint of ozone generation, pin or needle electrodes are preferred since they generate substantially less ozone. First attempts at utilizing pin electrodes for uniformly charging a charge retentive surface such as a photorecetor or plate negatively revealed that while they produce much less ozone than wire devices they did not provide the desired uniformity of charging.
The corona output from a needle-like array configuration can be especially non-uniform and very time dependently unstable at low operating in currents, especially when operating in the negative d.c. mode. For low plate current applications, this has required device designs which produce high shield currents in order to achieve reasonable uniformity with a practical device. Since ozone generation rate is well known to be proportional to the total output current such devices not only are inefficient but the ozone output thereform is substantially increased. Some reduction of the required inefficiency has been achieved by choosing various small radiused (less than 0.025 millimeter) needles for negative charging applications. However, even with sharp needles, it is still necessary to provide about 4 microamps per needle in a practical device in order to achieve reliable output stability and uniformity even though the required plate current is low.
In many perferred design configurations, it is desirable to increase the number of pins per area to be charged, and this decreases the current per pin at any given required operating device plate current. This results in even further decrease in efficiency being required in order to achieve stable output from the device. In other words, if the number of pins were doubled for a given plate current requirement then the current per pin would be half of what it was initially, i.e. with only half the pins. Assuming that the current per pin with the original number of pins was just adequate to produce stable outputs therefrom then the total current would have to be doubled in order to maintain stable output current from each pin. For this situation, more current would be drawn by the conductive shield. Thus, more ozone would be generated because more current is needed and the device would be less efficient because a larger portion of the total current to the pins flows to the shield for the new set of conditions.
It should now be apparent that an acceptable pin charging device must supply the required plate current and the current to each pin must be sufficient to produce stable outputs from the pins. Heretofore, in providing sufficient pin current to insure stable output therefrom large amounts of current were provided thus producing unacceptable ozone levels and a wasting of energy. One example of this is, as mentioned hereinabove, where the number of pins are increased for a given plate current requirement. Another example is where a higher current value is supplied to each pin for a particular pin design (i.e. a specific tip radius) than is required for stable output therefrom in order to compensate for manufacturing tolerances. Since it is quite difficult for every pin of an array to have exactly the same radius when manufactured it is desirable to provide current in excess of that required for the pin tip radius as per the design. In this way if one or more pins of the array have a slightly larger radius thereby requiring more current than the smaller radiused pin for stable output then the current needs for the larger radiused pin will be met. Even if the pin array could be manufactured so that each and every pin tip had exactly the same radius we have found that the tip geometry (i.e. tip radius) changes due to wear, chemical growth and toner contamination can be partially compensated for by supplying excess (i.e. more current than needed for stable output) current to each pin. However, as will be appreciated, when this is done using conventional power supply techniques, ozone generation is higher and the device is less efficient.